Power Plant Engineering

 Fig. 7.5. Bradford Breaker.
Pulverizers. The pulverizing process is composed of several stages. The first is the feeding
system, which must automatically control the fuel-feed rate according to the boiler demand and the air
rates required for drying (below) and transporting pulverized fuel to the burner (primary air). The next
stage is drying. One important property of coal being prepared for pulverization! is that it be dry and
dusty. Because coals have varying quantities of moisture and in order that lower-rank coals can be used,
dryers are an integral part of pulverizing equipment. Part of the air from the steam-generator air preheater,
the primary air, is forced into the pulverizer at 650°F or more by the primary air fan. There it is mixed
with the coal as it is being circulated and ground.
The heart of the equipment is the pulverizer, also called grinding mill. Grinding is accomplished
by impact, attrition, crushing, or combinations of these. There are several commonly used pulverizers,
classified by speed:
(1) low-speed: the ball-tube mill
(2) medium-speed: the ball-and-race and roll-and-race mill
(3) high-speed: the impact or hammer mill, and the attrition mill.
The low-speed ball-tube mill, one of the oldest on the market, is basically a hollow cylinder with
conical ends and heavy-cast wear-resistant liners, less than half-filled with forged steel balls of mixed
size. Pulverization is accomplished by attrition and impact as the balls and coal ascend and fall with
cylinder rotation. Primary air is circulated over the charge to carry the pulverized coal to classifiers
(below). The ball tube mill is dependable and requires low maintenance, but it is larger and heavier n
construction, consumes more power than others, and because of poor air circulation, works less effi-
ciently with wet coals. It has now been replaced with more efficient types
The medium-speed ball-and-race and roll-and race pulverizers are the type in most use nowa-
days. They operate on the principles of crushing and attrition. Pulverzation takes place between two
surfaces, one rolling on top of the other. The rolling elements may be balls or ring-shaped rolls that rolls

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FUELS AND COMBUSTION
227
that roll between two races, in the manner of a ball bearing. Fig. 7.6 shows an example of the former.
The balls are between a top stationary race or ring and a rotating bottom ring, which is driven by the
vertical, aft of the pulverizer. Primary air causes coal feed to circulate between the grinding elements,
and when it becomes fine enough, it becomes suspended in the air and is : carried to the classifier.
Grinding pressure is varied for the most efficient grinding of various coals by externally adjustable
springs on top of the stationary ring. The ball- and race pulverizer has ball circle diameters varying
between 17 and 76 in and capacities between 1J to 20 tons/h. The roll-and-race pulverizer is operated at
lower speeds and larger sizes. A typical one has an 89-in ball circle diameter, a 12 ft diameter, and a 22.5
ft height overall, weighs 150 tons, and is driven by a 700 hp motor. 3 oth types are suitable for direct-
firing systems (see below).
High-speed pulverizers use hammer beaters that revolve in a chamber equipped with high-wear-
resistant liners. They are mostly used with low-rank coals with high-moisture content and use flue gas
for drying. They are not widely used for pulverized coal systems.
Burner Pipe
Shutoff
Valve
Coal-and-Air
Discharge Openings
(individual Burner Lines)
Classifier
Raw
Coal
Feeder
Air Seal
Driving
Ring
Pyrites
Trap
Windbox